Re: [PATCH] fb: avoid possible deadlock caused by fb_set_suspend

Bruno Prémont
2011-06-18T08:43:38Z

On Fri, 17 June 2011 Florian Tobias Schandinat <FlorianSchandinat@gmx.de> wrote:
> From: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
>
> A lock ordering issue can cause deadlocks: in framebuffer/console code,
> all needed struct fb_info locks are taken before acquire_console_sem(),
> in places which need to take console semaphore.
>
> But fb_set_suspend is always called with console semaphore held, and
> inside it we call lock_fb_info which gets the fb_info lock, inverse
> locking order of what the rest of the code does. This causes a real
> deadlock issue, when we write to state fb sysfs attribute (which calls
> fb_set_suspend) while a framebuffer is being unregistered by
> remove_conflicting_framebuffers, as can be shown by following show
> blocked state trace on a test program which loads i915 and runs another
> forked processes writing to state attribute:
>
> Test process with semaphore held and trying to get fb_info lock:

...

> fb-test2 which reproduces above is available on kernel.org bug #26232.
> To solve this issue, avoid calling lock_fb_info inside fb_set_suspend,
> and move it out to where needed (callers of fb_set_suspend must call
> lock_fb_info before if needed). So far, the only place which needs to
> call lock_fb_info is store_fbstate, all other places which calls
> fb_set_suspend are suspend/resume hooks that should not need the lock as
> they should be run only when processes are already frozen in
> suspend/resume.

>From a quick look through FB drivers in 2.6.39 I've found one that would need
more work:
- drivers/video/sh_mobile_hdmi.c: sh_hdmi_edid_work_fn()
Should get changed to
a) right locking order in case (hdmi->hp_state == HDMI_HOTPLUG_CONNECTED)
b) lock fb_info in the other case
For this one fb_set_suspend() does get call in a hotplug worker,
thus independently on suspend/resume process.

The rest does match the suspend/resume hook pattern mentioned.

Bruno

> References: https://bugzilla.kernel.org/show_bug.cgi?id=26232
> Signed-off-by: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
> Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
> Cc: stable@kernel.org
> ---
> drivers/video/fbmem.c | 3 ---
> drivers/video/fbsysfs.c | 3 +++
> 2 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
> index 5aac00e..ad93629 100644
> --- a/drivers/video/fbmem.c
> +++ b/drivers/video/fbmem.c
> @@ -1738,8 +1738,6 @@ void fb_set_suspend(struct fb_info *info, int state)
> {
> struct fb_event event;
>
> - if (!lock_fb_info(info))
> - return;
> event.info = info;
> if (state) {
> fb_notifier_call_chain(FB_EVENT_SUSPEND, &event);
> @@ -1748,7 +1746,6 @@ void fb_set_suspend(struct fb_info *info, int state)
> info->state = FBINFO_STATE_RUNNING;
> fb_notifier_call_chain(FB_EVENT_RESUME, &event);
> }
> - unlock_fb_info(info);
> }
>
> /**
> diff --git a/drivers/video/fbsysfs.c b/drivers/video/fbsysfs.c
> index 04251ce..67afa9c 100644
> --- a/drivers/video/fbsysfs.c
> +++ b/drivers/video/fbsysfs.c
> @@ -399,9 +399,12 @@ static ssize_t store_fbstate(struct device *device,
>
> state = simple_strtoul(buf, &last, 0);
>
> + if (!lock_fb_info(fb_info))
> + return -ENODEV;
> console_lock();
> fb_set_suspend(fb_info, (int)state);
> console_unlock();
> + unlock_fb_info(fb_info);
>
> return count;
> }